This invention relates to a method for cementing a casing in a wellbore in which the casing will subsequently be heated to a high temperature, and more particularly to a method for anchoring the lower end of the casing while upward tension is applied to the casing while cement is allowed to set.
Prior art, which is believed to be relevant to the present invention, includes U.S. Pat. No. 3,976,139, issued to L. B. Wilder on Aug. 24, 1976, and assigned to the assignee of the present invention. Discussed in this patent are the problems encountered in thermal wells in which heating of the casing can cause buckling and joint failure. This is due to the fact that in normal cementing operations the tension applied to the casing is merely that occurring due to its own weight. This tension has been found to be insufficient to prevent the casing from being placed in a condition of compression at high temperatures which are often encountered. As discussed in the patent, such problems can be overcome by anchoring the bottom of the casing during the cementing operation and applying extra tension to the casing while the cement sets. The patent discusses prior art mechanical anchors which have been used and discloses an improved anchor. The disclosed anchor has a number of steel arms which are extended from the casing after cement has been circulated through the annulus and engage the borehole wall to resist upward movement of the casing. As noted at column 4, lines 44-47, of that patent, the anchors tend to plow through soft formations when they are encountered. It has been found in many cases that formations may be in fact so soft or the borehole is so irregular that the anchors will not provide sufficient resistance to movement to allow the desired level of extra tension to be applied. As a result, it has become standard practice to use fast-setting cement at the bottom of the borehole so that it may be used as an additional anchor while slower-setting cement in the upper parts of the borehole is still fluid as tension is placed on the casing.
It can be seen from an inspection of the prior art that these mechanical anchoring devices have been, in general, fairly complex. Even with such complexity, they have been found to not provide the desired anchoring in soft formations or irregular shaped boreholes.
Accordingly, an object of the present invention is to provide an improved and simplified method of extra tensioning a casing string during cementing operations.
Another object of the present invention is to provide a method of extra tensioning a casing string which is particularly effective in soft formations or irregular shaped boreholes.
These and other objects are achieved by using an inflatable casing packer at the bottom of the casing string as an anchor. Cement is pumped behind the casing in the normal manner prior to inflation of the packer. A cement wiper plug is then used to actuate the inflatable packer which is inflated with additional cement pumped in behind the wiper plug. After the packer is inflated with the desired pressure, extra tension is applied to the casing while the cement is allowed to cure.